IN THE NAME OF GOD

SUBJECT:

Hydrolytic Degradation

Hydrolytic Degradation:

This process occurs in polymers that are water-sensitive active groups, especially those that take a lot of moisture.

Polymers that have an ability for hydrolytic destruction usually have heteroatoms in the main or side chain.

hydrolysis :the scission of chemical functional groups by reaction with water. some polymers are very stable to hydrolysis

:Water Absorption Plastics absorb water to varying

degrees, depending on their molecular structure, fillers and additives

Adversely affects both mechanical and electrical properties and causes swelling

Measures the amount of water absorbed as a percent of total weight

Exposing plastics to moisture at elevated temperature can lead to hydrolysis A chemical process that severs polymer

chains by reacting with water Reduces the molecular weight and

degrades the plasticDegree of degradation depends on

Exposure time Temperature Stress levels

degradation rate dependent on hydrophobicity crystallinity Tg

impurities initial molecular weight, polydispersity degree of crosslinking manufacturing procedure geometry site of implantation

Hydrolytic Degradation can also be harmful and helpful in cases, for example:

Hydrolytic chain scission of the ester linkage of polyester is the fastest and the most “harmful” degradation process, as it causes a considerable reduction in the molecular weight and in the mechanical properties of the polymer. but The polymer degradation useful for Controlled Drug Delivery System

For most biodegradable materials, especially artificial polymers, passive hydrolysis is the most important kind of degradation . There are several factors that influence the velocity of this reaction; the most important are: type of chemical bond PH copolymer composition water uptake morphology

:Hydrolysis tests The hydrolytic degradation test was

performed in a basic aqueos solution at pH 12.7 at 50 °C. Each sample was in kept in a sealed container with 50 ml of solution. The degradation media was changed regularly so that the pH was maintained and polymer matrix samples were taken after time intervals. Degraded specimens were collected at desired intervals and dried for 24 h in a vacuum oven at room temperature, and then the weight loss was measured

Hydrolytic Degradation Behavior of Biodegradable Polymers from Controlled Drug Delivery System: There are different types of polymer degradation

such as photo, thermal, mechanical and chemical degradation. For in-vivo application; thermal degradation is not of much significance. biodegradable polymers contain hydrolysable bonds making them more prone to chemical degradation via hydrolysis or enzyme-catalyzed hydrolysis. Enzymatic degradation does not play significant role in polymers belonging to lactide/glycolide family. Therefore, study of hydrolytic degradation is utmost important while considering performance of polymeric implants or polymeric drug delivery system.

Mechanism of Polymer Degradation and Erosion: The difference between polymer

“degradation“ and “erosion“ is not clear in many cases. Biodegradable polymers undergo hydrolytic bond cleavage to form water soluble degradation products that can dissolve in an aqueous environment, resulting in polymer erosion . In this context, degradation is a chemical phenomenon and erosion encompasses physical phenomena, such as dissolution and diffusion. Polymer degradation is the key route of erosion.

Polymer erosion is so far more complex than degradation, because it depends on many other processes, such as degradation, swelling, dissolution and diffusion of oligomers and monomers, and morphological changes. The erosion of a polymer matrix can proceed through two alternative physical mechanisms: (a) surface erosion and (b) bulk erosion. For ideal surface erosion, erosion rate is constant and proportional to external surface area. For bulk eroding polymers such as PLA and PLGA, things are more complicated as they have no constant erosion rate

One is homogeneous or bulk erosion, in which hydrolysis occurs simultaneously throughout the entire specimen.Hence, the decrease in molecular weight, the reduction in mechanical properties, and the loss of mass also occur throughout the entire specimen at the same pace. Polymers containing ester, ether, and amide groups, such as poly(lactic acid) (PLA), poly(glycolic acid), poly(ε-caprolactone), polyamide, proteins, and cellulose (and its derivatives), generally exhibit this type of erosion.

These reactions can sometimes be catalyzed by the polymers’ own hydrolysis products. The other type is heterogeneous or surface erosion, in which hydrolysis mainly occurs in the region near the surface, whereas the bulk material is only slightly or not at all hydrolyzed. As the surface is eroded and removed. Polymers such as poly(ortho ester)s (POEs), PAHs, and some polycarbonates tend to undergo surface erosion.

:Degradation Mechanism of Hydrolytic

Hydrolytic degradation of members of the polylactide/glycolide family proceeds through four stages as represented in Figure 2: First stage of water diffusion followed by second stage, in which oligomers with acidic end-groups autocatalyze the hydrolysis reaction. A critical molecular weight is reached at the beginning of third stage,and oligomers start to diffuse out from the polymer. Water molecules diffuse into the void created by the removal of the oligomers, which in turn encourages oligomers diffusion.

Marked decrease in polymer mass and a sharp increase in the drug release rate occur during third stage as the drug diffuses from the porous regions. In fourth stage, polymeric matrix become highly porous and degradation proceeds homogeneously and more slowly .

Factors Influencing Polymer Degradation: In recent years a number of parameters have

been identified that influence the polymer degradation. Among them are the copolymer composition , morphology, autocatalysis by acidic degradation products inside a matrix , presence of drugs or other excipients and preparation technique. However, the impacts of these parameters that increase or decrease the degradation rate are not exactly clear. Review on effect of various factors on polymer degradation is presented in Table 1.

Hydrolytic Degradation Behavior of Poly(butylenesuccinate)s with Different Crystalline Morphologies

The effects of crystallinity and morphology on polymer hydrolysis were studied for a decade. It is known that hydrolytic degradation proceeds at a higher rate in the amorphous region than in the crystalline region. The higher degradation rate in the amorphous region is attributed to the easy diffusion of water molecules into the interior of polymers.

Techniques Used to Study Polymer Degradation:

Hydrolytic degradation and erosion were then followed through time by FTIR spectroscopy and weight loss measurements. In the solid state, titration of acid end group, infrared (IR) spectroscopy and intrinsic viscosity were used to analyze the hydrolytic degradation in order to define the degradation rate.

REFERENCES: http://www.sbaoi.org. CHARACTERIZATION AND DEGRADATION

MECHANISMS OF ALIPHATIC POLYESTERS. Degradation as a Method of Modification of

Polymeric Products. From Wikipedia, the free encyclopedia www.sciencedirect.com